If your hair is a mess but you don't have time to shower, dry shampoo is a great alternative to soak up the oils that make your hair greasy. Usually these are sold as aerosols, but you can make your own powder version with a few household ingredients.
jessyratfink shares two versions of the shampoo on Instructables: one for light hair and one for dark. For light hair, just combine 1/4 cup of cornflour and a tablespoon of bicarbonate soda in a spice jar. For dark hair, add in two tablespoons of unsweetened cocoa powder.
Once everything's mixed together, sprinkle a small amount in your hands, rub it into your hair, let it sit for a moment, then brush it out. The powder should absorb a lot of oil and make your hair look and feel less greasy.
Homemade Dry Shampoo [Instructables]